Platelet derived growth factor (PDGF) is a mitogen and chemoattractant for cells of mesenchymal origin, such as fibroblasts, smooth muscle cells, and glial cells. This cytokine was first described as a platelet-derived mitogenic activity in serum, although subsequent studies have revealed that a variety of cells, including macrophages, endothelial cells and several tumor cell types also secrete the cytokine. PDGF is encoded by two genes, the products of which are designated A and B. The active PDGF molecule is a disulfide-linked dimer of these polypeptides, and thus can exist in three forms: the homodimers AA or BB, or the heterodimer AB. Platelets synthesize 70% AB, 20% BB and 10% AA. The A and B polypeptides exhibit a 60% homology at the amino acid sequence level, with complete conservation of eight cysteine residues.
Studies examining the interaction of PDGF with responsive cells have revealed the existence of two specific receptors, designated .alpha. and .beta.. Each receptor type is composed of five extracellular antibody-like domains attached to an intracellular tyrosine kinase domain via a transmembrane segment. The molecular weights of the mature forms of the PDGF receptors are 170,000 and 180,000 Daltons for the .alpha. and .beta. forms, respectively. The receptors are heavily glycosylated; carbohydrates on the extracellular portion of the receptors account for approximately 30% of the total molecular mass.
The active form of the PDGF receptor (PDGF-R) is a dimer, with two receptor molecules being bridged by a single PDGF dimer. The different forms of PDGF exhibit different affinities for the two forms of the PDGF receptor. PDGF-R .alpha. can interact with all three forms of PDGF; PDGF-R .beta. can only interact with PDGF BB and AB. This pattern of reactivity dictates that PDGF AA can signal the cell only through homodimers of PDGF-R .alpha.; PDGF BB can signal the cell through homodimers of PDGF-R .alpha. or PDGF-R .beta., or the heterodimer of PDGF-R .alpha. and PDGF-R .beta.; and PDGF AB can stimulate cells through either homodimers of PDGF-R .alpha., or the heterodimers. Claesson-Welsh, J. Biol. Chem. 269:32023 (1994).
Antibodies specific for PDGF-R have been isolated. Early antibodies had little utility. Kawahara et al. (1987) Biochim. Biophys. Res. Commun. 147:839-845. Additional monoclonal antibodies have been raised against the extracellular PDGF-binding domain of PDGF-R from porcine uterus but these antibodies did not inhibit binding of .sup.125 I-labeled PDGF to human fibroblasts. Ronnstrand and Terracio (1988) J. Biol. Chem. 263:10429-10435. Numerous antibodies specific for a PDGF-R have been isolated but did not inhibit PDGF activity. Kanakaraj et al. (1991) Biochem. 30:1761-1767; Claesson-Welsh et al. (1989) J. Biol. Chem. 264:1742-1747; Seifert et al. (1989) J. Biol. Chem. 264:8771-8778; Kumjian et al. (1989) Proc. Natl. Acad. Sci. USA 86:8232-8236; Bishayee et al. (1988) Mol. Cell Biol. 8:3696-3702; Hart et al. (1987) J. Biol. Chem. 262:10780-10785; Escobedo et al. (1988) J. Biol. Chem. 263:1482-1487; Daniel et al. (1987) J. Biol. Chem. 262:9778-9784; Keating and Williams (1987) J. Biol. Chem. 262:7932-7937; and Kazlauskas and Copper (1990) EMBO J. 9:3279-3286. More recently, antibodies that inhibit the effects of PDGF have been described. WO 93/10805; and WO 94/19016. The antibodies described in WO 93/10805 bind to the fifth Ig-like domain of PDGF-R .beta.. WO 94/19016 describes the use of antibodies specific for PDGF-R .alpha. and .beta. to reduce intimal hyperplasia.
As a potent mitogenic and chemotactic agent, PDGF has been implicated as a contributing factor in a number of pathologic conditions that involve the migration and proliferation of PDGF responsive cells. Such conditions include arteriosclerosis, restenosis following coronary bypass surgery or balloon angioplasty, nephritis, scleroderma, and some types of cancer including lung cancer and Kaposi's sarcoma.
Restenosis is a common complication of balloon angioplasty or coronary bypass surgery, occurring after up to 40% of such procedures. Holmes et al. (1988) J. Amer. Coll. Cardiol. 12:1149. Histologically, the restenosis is due almost entirely to the accumulation and proliferation of neointimal smooth muscle cells, i.e., to intimal hyperplasia. PDGF is both chemotactic and mitogenic for smooth muscle cells, suggesting that this cytokine is involved in causing restenosis. Experimental support for a connection of PDGF to restenosis exists. Treatment of rats with a goat polyclonal antiserum raised against human PDGF (all isoforms) resulted in a 40% decrease in subsequent neointima formation following vascular de-endothelialization via a balloon catheter. Ferns et al. (1991). Thus, neutralizing the PDGF-mediated mitogenesis of smooth muscle cells appears to be a viable therapeutic tactic for this complication.
All references cited herein, both supra and infra, are hereby incorporated herein by reference.